Solution to practical exercise 2

1
Solution topractical exercise 2

• Thread synchronization in Java
• The Producer/Consumer model
• The Barbershop example

2
The exercise

• Your task was to implement a version of the
  Barbershop example
• A doorman produces customers.
• Customers are buffered in a ring of chairs.
• Three barbers consume customers.
• A handed-out GUI visualizes the proceedings.

3
Key points

• The buffer of chairs is implemented by the
  CustomerQueue class.
• This class serves as a monitor.
• The Barber and Doorman classes implement the
  Runnable interface, and have run() methods
  consuming and producing customers, respectively.

4
The run() methods

• The Doorman repeatedly creates new customers and
  inserts them into the CustomerQueue
• public void run ()
• while(!stopRunning)
• // Sleep a random period, then add a new
  customer
• try
• Thread.sleep(
• (long)(Math.random()Globals.doormanSleep)
  )
• catch (InterruptedException e)
• customerQueue.addCustomer(new Customer())

5
The run() methods

• The Barber repeatedly removes customers from the
  CustomerQueue
• public void run()
• while(!stopRunning)
• // Sleep a random period
• try
• gui.barberIsSleeping(myPosition)
• Thread.sleep((long)(Math.random()Globals.ba
  rberSleep))
• gui.barberIsAwake(myPosition)
• catch (InterruptedException e)
• // Serve a new customer
• Customer c customerQueue.removeCustomer(myPo
  sition)
• serveCustomer(c)
• The stopRunning variable is set to true by the
  stopThread() method and is used to end the thread
  in a safe way.

6
The buffer

• The customer buffer is implemented by the
  CustomerQueue class and takes care of thread
  synchronization.
• The CustomerQueue class encapsulates a data
  structure with three variables that must be
  protected by a monitor
• / The circular buffer of customers /
• private Customer buffer
• / The current position of the first customer in
  the queue /
• private int head
• / The current pos. of the first free place in
  the queue /
• private int tail
• If operations on these variables are not mutually
  exclusive, data inconsistencies can easily occur.

7
Adding customers

• The addCustomer() method adds a customer to the
  buffer,
• but must wait if the buffer is full.
• public synchronized void addCustomer(Customer c)
  
• while(currentLength gt maxLength)
• gui.println("Doorman is waiting for free
  chairs...")
• try
• wait()
• catch (InterruptedException e)
• gui.println("Doorman was notified of a free
  chair.")
• // Add the customer at the tail of the queue
• buffertail c
• // Update the GUI
• gui.fillLoungeChair(tail, c)
• // Update the queue parameters
• tail (tail1)maxLength
• currentLength
• // If the queue was empty, notify all the
  barbers

8
Removing customers

• The removeCustomer() method removes a customer
  from the buffer,
• but must wait if the buffer is empty.
• public synchronized Customer removeCustomer(int
  barberNo)
• while(currentLength 0)
• gui.println("Barber "barberNo" is waiting
  for customers...")
• try
• wait()
• catch (InterruptedException e)
• gui.println("Barber "barberNo" was
  notified of a new customer.")
• // Fetch the customer at the head of the queue
• Customer c bufferhead
• // Update the GUI
• gui.emptyLoungeChair(head)
• // Update the queue parameters
• head (head1)maxLength
• currentLength--
• // If the queue was full, notify the doorman
  that there's a free chair

9
The message flow

• When a doorman invokes addCustomer(), and the
  buffer was empty, he notifies all barbers that
  are waiting for customers that a customer has
  been added.

public synchronized void addCustomer(Customer
c) // If the queue was empty, notify
// all the barbers that a customer // has
arrived if(currentLength 1)
notifyAll()
public synchronized Customer
removeCustomer() while(currentLength 0)
try wait()
catch(InterruptedException e)
10
The message flow

• When a barber invokes removeCustomer(), and the
  buffer was full, he notifies the doorman that
  there is room for more customers.

public synchronized Customer
removeCustomer() // If the queue was
full, notify //the doorman that there's a
//free chair if (currentLength
(maxLength-1)) notifyAll() return c
public synchronized void addCustomer(Customer
c) while(currentLengthgt maxLength)
try wait() catch(InterruptedExce
ption e)
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Synchronization logic

• Note that a notifyAll() call will wake up all
  waiting threads, also threads who are waiting at
  the wrong place.
• It is up to the programmer to ensure that threads
  wait at the right place.
• In this case, the buffer cannot be both empty and
  full, so there cannot be threads waiting in the
  removeCustomer() and addCustomer() methods
  simultaneously.
• If these kinds of guarantees cant be made, each
  pair of wait() and notify() invocations must use
  separate monitors.

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Questions?